1. Field of the Invention
The present invention relates to a display device, and more particularly to a display device using a substrate which is liable to be deformed two-dimensionally such as a plastic substrate or a thin glass substrate.
2. Description of the Related Art
Various kinds of display devices such as a liquid crystal display device and an organic EL display device have been put into practice. Further, mobile phones and personal digital assistants which incorporate these display devices also have been provided. In display devices developed and used in recent years, to enhance the reduction of thickness, to enhance the reduction of weight or to increase the flexibility of the display device, a substrate which constitutes a part of the display device is formed using a plastic substrate or a thin glass substrate.
With respect to a display device which uses a plastic substrate, as disclosed in JP-A-2002-184959 (patent document 1) or the like, there has been proposed a flexibly deformable display device in which functional elements such as thin film transistors (TFTs) are formed on a glass substrate and, thereafter, glass is removed or the glass substrate is formed into a thin film, and functional elements are adhered to (or transferred to) a plastic substrate.
However, to use the plastic substrate in the display device such as a liquid crystal display device or an organic EL display device, the plastic substrate is required to possess high size stability (low thermal expansion property or high heat resistant property) and hence, the plastic substrate exhibits characteristics that the plastic substrate exhibits high resistance against bending but is liable to be broken when elongation deformation is applied to the plastic substrate. In fact, when a liquid crystal display panel which uses a plastic substrate is deformed, the liquid crystal display panel has a drawback that although the liquid crystal display panel exhibits high flexibility with respect to one-dimensional bending (one-dimensional deformation) such as a four-point bending test, the liquid crystal display panel is liable to be broken with respect to a point load which easily causes the two-dimensional bending (two-dimensional deformation) of the liquid crystal display panel such as a load falling test. Even when the liquid crystal display panel uses a thin glass substrate which is made thin enough to be bendable, in the same manner as the plastic substrate, the liquid crystal display panel exhibits more brittleness against the two-dimensional deformation than one-dimensional deformation.